Your task is to create a program that prints "Greetings, Planet!" exactly to the console. Seems simple enough, right? Well, here's the trick. The code must be a palindrome, meaning it reads from right to left the same as in left to right.
Standard loopholes are NOT allowed.

\$\begingroup\$This does not seem to be "hello world".\$\endgroup\$
– Erik the OutgolferMay 18 '16 at 13:43

4

\$\begingroup\$@ΈρικΚωνσταντόπουλος It's the same as Hello World for all intents and purposes. The goal is to print a short string consisting of two words with the punctuation common in Hello World programs as well. Apart from built-ins that print Hello World, solutions will be absolutely identical (apart from the actual string) regardless of the actual string being printed. (And I assume the phrase "Greetings, Planet!" was chosen specifically to make built-ins useless.)\$\endgroup\$
– Martin EnderMay 18 '16 at 13:52

1

\$\begingroup\$@MartinBüttner I think that Hello, world! and Hello World! are the only outputs allowed for a hello-world program.\$\endgroup\$
– Erik the OutgolferMay 18 '16 at 14:09

These answers are a bit different to every other; there is a chance for either direction of code to be executed.

So, by "printing to console", I assumed you meant printing to stdout. This throws an error; the error is thrown to stderr AFTER the string is printed to stdout.

To prove that this worked both ways, I used the "random direction" director, "x". Fish is a two-dimensional language, so, no matter which way the director points, the code will still (eventually) be executed.

Assuming that the first director points to the right, the characters are loaded to the "stack" in reverse order, then the reverse of the reverse (or the normal text) is printed.

Assuming that both directors point to the left, the characters are, once again, loaded to the "stack" in reverse order (because the code loads it in backwards here, direction is to the left), then the reverse of the reverse (or the normal text) is printed.

If the randomized director points up or down, this won't matter - fish knows to loop to the underside or overside of the code, pointing back to the randomizer. In this way, it will continue to loop with the randomizers until it points inward, towards the code to execute.

The !|o|! bit does the following, from both sides:

! skips the next instruction (will always skip |)

| is a reflector; it points inward back towards o.

o outputs the current item of the stack to console as a character and removes it from the stack.

So, essentially, this is the "two mirrors in a bathroom pressed up together" trick, where I output until I can't anymore.

Now using a cat emoji. >o< redirects the output inward infinitely, still throwing the error, but I can get away with not using a skip into reflection.

Turns out I was right the first time - the second attempt was not palindromic, but it was reflective.

Fish (without printing to stderr), 64 Bytes (ew)

x"!tenalP ,sgniteerG"!;oooooooooooooooooo;!"Greetings, Planet!"x

This dude's a little longer.

It has the same randomized arrow function (to prove it works both ways) and does not print to stderr.

The difference here is obvious; I literally print out every item in the stack, then end execution with ;.

The !; does not end execution immediately, as ! skips the next item (end exec, in this case) and continues until it hits the other side, which acts as ;!, wherein it ends execution before it skips anything.

It follows the same randomized direction pattern as the shorter answer.

\$\begingroup\$as i guess , it's unpridictable wheather the full code will be excuted or not . What if program counter is stuck between two random direction instructions , or just stuck at one of the random direction instructions , for example , it turns down , reachs the bottom , comes back to that instruction and start all over ?\$\endgroup\$
– user55673Jul 13 '16 at 17:11

\$\begingroup\$@GLASSIC The program will only output once, and the entire code will never be executed.\$\endgroup\$
– Addison CrumpJul 14 '16 at 3:18

Jelly, non-competing

How it works

»1ị“RsẈḄ,#ʠU“Uʠ#,ḄẈsR“ị1» Main link. No arguments.
“ “ “ » Decompress all three strings; yield a list of strings.
RsẈḄ,#ʠU (decompresses to 'Greetings, Planet!')
Uʠ#,ḄẈsR (decompresses to ' WafdkmC Posited,')
ị1 (decompresses to 'Taarhus')
»1 Take the maximum of the default argument (0) and 1.
ị Select the string at the index to the left.

\$\begingroup\$FYI, a lot of your answers (this one included) have been getting tossed in the Low Quality queue because they're solely a title/score and code. Though there's nothing wrong with that, you may consider adding more text (such as an explanation) so they don't get auto-flagged.\$\endgroup\$
– Mego♦Nov 29 '15 at 2:57

\$\begingroup\$I'm not saying the lack of an explanation is a bad thing (though answers with explanations tend to get more upvotes), I'm just letting you know that your (valid) answers were getting auto-flagged because of their length. :)\$\endgroup\$
– Mego♦Nov 30 '15 at 9:12

R initialises an atom which moves to the right. " toggles string mode which simply prints the desired string to STDOUT before hitting ;, which destroys the atom and terminates the program. The first half is simply never executed.

STATA, 52 bytes

di "Greetings, Planet!"//"!tenalP ,sgniteerG" id

A slightly longer (53 byte) version that doesn't use comments is:

#d
di "Greetings, Planet!";"!tenalP ,sgniteerG" id
d#

#d [something] changes the delimiter (initially a carriage return) to ; unless [something] is cr, so the first command changes the delimiter to ;, the second prints the string, and the third (which continues until the end) is apparently a nop, though I have no idea why. I would have guessed that this would throw an error (unrecognized command "!tenalP ,sgniteerG" or something), but apparently not.

TCL, 80 Bytes

explanation: TCL executes a global proc unknown when it encounters a call to an undefined command, the first line redefines that proc to a simple "hello world" program.

TCL's quoting rules are quite subtle, an open brace starts a quoted word that extends to the next matching close brace, allowing nested, quoted words. Braces are otherwise treated as normal characters. five words: }"!dlroW olleH", stup{, sgra, nwonknu and corp. No command named }"!dlroW olleH" has been defined, so the undefined proc from the first line is invoked instead.

A similar question was posted on the StackOverflow of antiquity; which has since been closed and deleted. I used my solution as a tongue in cheek TCL sample in this answer, and am now getting comments asking for an explanation, so I recreate my answer here.

The first half just before the middle , builds a reverse greeting string including the additional G on the very right. The middle , drops it and the rest is a canonical "Hello/Greeting/whatever" program. The additional characters on both ends are needed, because the message contains a ,. The left one is executed and drops the s. Both , and s need a replacement for the left concatenation dots, hence the Gs. The , on the right isn't executed, but build into the final string.

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